Numerous articles of commerce are manufactured by subjecting a moldable composite sheet to a molding process. The moldable composite sheets usually comprise a thermoplastic resin such as a polypropylene resin which is reinforced with reinforcement fibers such as a glass fiber mat. These composite sheets are commonly referred to as glass mat thermoplastics or GMT. These composite sheets can be heated to soften the resin and molded under pressure to make various articles such as pans for automobiles, cabinetry for computers and other electronic equipment, and components for small appliances and other consumer and industrial goods. A typical molding process for these composite sheets is compression molding in which the moldable composite sheet is heated, pressed into a mold, and subjected to pressure in a relatively cold mold to form the molded part. The resin and reinforcement material flow during the molding process to fill out the entire volume of the mold.
A critical aspect of molded composite products is the product mechanical properties. Measured properties typically include flexural strength/modulus, tensile strength/modulus, and impact strength. The amount and type of reinforcing fibers in the composite greatly affect the mechanical properties of the product. Generally, in a glass fiber reinforced composite material, the higher the glass/resin ratio, the stronger the product. However, since the fiberglass reinforcements are more expensive on a weight basis than the resin, composite manufacturers have sought to design moldable composite sheets having as low a glass/resin ratio as possible while still meeting target mechanical properties. Also, specific fiber surface coatings or sizes have been developed to increase the glass fiber strength and handleability, and to enhance the bonding of the glass fiber reinforcement to the resin matrix.
One widely used moldable composite sheet is available from Azdel, Inc., Shelby, N.C. The Azdel.RTM. moldable composite sheet uses a needled continuous strand mat combined with polypropylene. The glass fiber strands in the continuous strand mat are generally long, and each of the strands contains numerous filaments, with generally at least 50 filaments per strand. The Azdel composite sheet is formed by extruding three continuous sheets of resin interspersed with two continuous strand mats. By using previously formed reinforcing mats in the process, impregnation of all of the filaments by the resin is very difficult, and the reinforcement fibers tend to be concentrated in two horizontal layers, and the reinforcements are not generally uniformly distributed throughout the vertical cross-section of the product.
Wiggins Teape Group Ltd, Basingstoke, England, has developed a process (the "Radlite.RTM." process) in which glass and polypropylene are mixed in a slurry, and then cast into sheets. The slurry mixing process necessarily causes fiber breakup, resulting in an average fiber length less than about 5 cm. Another widely used moldable composite sheet is the Taffen.RTM. sheet manufactured by Exxon Corporation, Houston, Tex. Exxon produces the Taffen.RTM. sheet with a process similar to the Radlite process by adding a thermoplastic polymer material (polypropylene) to a glass fiber wet process mat.
Composites Products Inc. (CPI), in Winona, Minn., uses a dual-screw apparatus to uniformly mix and compound long chopped glass fibers (up to about 3 cm) with resin to form a mass of composite material which can be molded. The glass fibers in the CPI product end up having an average length up to about 0.5 cm due to the forces applied to the fibers during the compounding process.
One of the problems of prior art GMT systems using a preformed mat, such as a continuous strand mat, is that a preformed mat of glass fibers is difficult to impregnate and wet out in a subsequent resination step. Some GMT manufacturers have tried to enhance impregnation by decreasing the porosity of the mat or increasing the length of the fibers, but the only way to maintain the glass loading required for the mechanical properties of the ultimate composite product is to group or bundle the reinforcement fibers into strands. This hinders the ability to provide a uniform distribution of the reinforcements within the resin matrix, since the bundles of glass fiber reinforcements are not easily dispersed.
It would be desirable to provide a moldable composite sheet having increased mechanical properties when compared to existing moldable composite products. Also, it would be advantageous if such an improved product would make the optimum use of the glass fiber reinforcement materials, which would mean that the reinforcing fibers would be uniformly distributed throughout the composite product rather than concentrated in a few specific horizontal layers.